President Donald Trump has signed a memorandum that bans most transgender individuals from serving in the US military.
The memorandum said that transgender individuals with a history of 'gender dysphoria' are disqualified from military service except under certain limited circumstances.
It defined such individuals as "those who may require substantial medical treatment, including through medical drugs or surgery".
The order added that the secretaries of defence and homeland security may exercise their authority to implement any appropriate policies concerning military service by transgender individuals.
The White House on Friday said that Defence Secretary Jim Mattis had found that individuals with a history or diagnosis of gender dysphoria presented a risk to military effectiveness.
"This new policy will enable the military to apply well-established mental and physical health standards... equally to all individuals who want to join and fight for the best military force the world has ever seen," the White House said.
Mr Trump announced in July that he would prohibit transgender people from serving in the military, reversing former President Barack Obama's policy of accepting them.
A number of federal judges have issued rulings blocking Mr Trump's ban. The judges said the ban would likely violate the right under the US Constitution to equal protection under the law.
On Friday, the Pentagon reaffirmed that it would continue to comply with federal law.